Pulping and Bleaching PSE 476/Chem E 471

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Pulping and BleachingPSE 476/Chem E 471

• Lecture 17
• Introduction to Bleaching

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Agenda

• Brightness
• General Bleaching Principles
• Chemistry
• Process
• Chemicals
• Description
• Advantages/Disadvantages

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Why Bleach?

• Improve brightness.
• Improve brightness stability.
• Clean up pulp (impurities).
• Wood based (bark, resins, sand, shives).
• Process based (carbon specs, rust, rubber).
• External sources based (plastics, grease, ash).
• Increase capacity of paper to accept printing.

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The purpose of bleaching
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Bleach plant
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Brightness Determination (1)
Light shinning on a sheet of paper is either
transmitted, adsorbed, or reflected.

• Light is scattered by fibers at air/fiber
  interfaces
• Light is adsorbed by certain chemicals in the
  fibers (lignin)

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Brightness Determination (2)

• Brightness is measurement of how much light is
  reflected from a sheet of paper.
• Whiteness does not mean brightness.
• Whiteness is a physical phenomena related to how
  the eye views the paper.
• A very white looking piece of paper may not have
  high brightness.
• Example blue dye added to a yellow tinged sheet
  of paper will give a white sheet of paper with
  low brightness.

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Brightness Determination (3)

• Brightness determination method
• Light reflectance measured and compared to light
  reflectance from MgO.
• MgO assumed to reflect 100 light.
• Brightness is reported as of MgO reflectance
  (85 brightness is equivalent to 85 of MgO).
• Variables
• Angle of light Light is applied to sheet at 45
  angle.
• Wavelength 457 nm (blue light most sensitive).
• Pine kraft
• Unbleached-ISO 23-28
• Semi bleached-ISO 60-80
• Bleached-ISO 88-91

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General Principles

• Two types of bleaching
• Lignin removing chemical pulps.
• Lignin retaining mechanical pulps.
• Bleaching is used because at a certain point in
  the pulping process, carbohydrate degradation
  becoming greater than lignin removal.
• Bleaching chemicals are more selective for
  lignin.
• Bleaching chemicals much more expensive than
  pulping chemicals so they are not used in
  pulping.

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General PrinciplesChemistry

• Pulping
• Pulping typically involves cleavage of ether
  linkages and some substitution (sulfonation).
• Bleaching
• Bleaching involves attacks on aromatic rings,
  olefinic structures, and carbonyl groups.
• Substitution reactions play a big role.

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Multiple stages of bleaching
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General PrinciplesProcess

• Bleaching uses a combination of chemicals in
  series.
• One chemical alone will not remove residual
  lignin.
• Each step reacts with material modified in
  previous step.

NaOH
NaOH
ClO2
ClO2
O2
O2
Unbleached
D
D
EO
EO
Bleached Pulp
Pulp
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Washing
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General PrinciplesChemicals (1)
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General PrinciplesChemicals (2)
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General PrinciplesChemicals (3a)
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General PrinciplesChemicals (3b)
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General PrinciplesChemicals (3c)
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Groups of bleaching chemicals (2)

• Bleaching chemicals can be divided into three
  groups according to their function
• 1 Group
• The chlorine (Cl2), ozone (O3) reacts with all
  aromatic lignin units (phenolic groups and their
  bonds)
• 2 Group
• The chlorine dioxide(ClO2) and oxygen (O2) reacts
  in general with lignin structures that have free
  phenolic hydroxyl groups
• 3 Group
• The hypochlorite (H) and hydrogen peroxide (H2O2)
  reacts only with certain functional groups, for
  example carbonyl groups

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Bleaching reactions

• Bleaching chemicals are used primarily as
  oxidants, to break down residual lignin and to
  increase its solubility.
• Mode of operation
• Electrophiles (oxidative reactions, low pH,
  involve cations)
• Nucelophiles (reductive reactions, high pH,
  anions)
• Radicals

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Bleaching Generalities

• It is important to note that when bleaching with
  a specific reagent, it will be converted into a
  number of different reactive species which will
  react with lignin and carbohydrates differently.
  A simple example is when chlorine gas is added
  to water both hypochlorous acid and/or
  hypochorite is formed depending on the pH.